High prevalence of many infectious diseases, such as invasive pneumococcal disease (IPD) and increasing antibiotic resistance of the related pathogens requires urgent development of protective vaccines. Especially as existing vaccines exhibit major drawbacks such as variable immunogenicity and the lack of development of immunological memory.
Vaccines have traditionally consisted of live attenuated pathogens, whole inactivated organisms or inactivated toxins. In many cases, these approaches have been successful at inducing immune protection based on antibody mediated responses. However, certain pathogens, e.g., HIV, HCV, TB, and malaria, require the induction of cell-mediated immunity (CMI). Non-live vaccines have generally proven ineffective in producing CMI. In addition, although live vaccines may induce CMI, some live attenuated vaccines may cause disease in immunosuppressed subjects.
In contrast to older vaccines which were typically based on live attenuated or non-replicating inactivated pathogens, modern vaccines are composed of synthetic, recombinant, or highly purified subunit antigens. Subunit-vaccines are designed to include only the antigens required for protective immunization and are believed to be safer than whole inactivated or live-attenuated vaccines. However, the purity of the subunit antigens and the absence of the self-adjuvanting immunomodulatory components associated with attenuated or killed vaccines often result in weaker immunogenicity.
The immunogenicity of a relatively weak antigen can be enhanced by the simultaneous or more generally conjoined administration of the antigen with an “adjuvant”, usually a substance that is not immunogenic when administered alone, but will evoke, increase and/or prolong an immune response to an antigen. In the absence of adjuvant, reduced or no immune response may occur, or worse the host may become tolerized to the antigen.
Adjuvants can be found in a group of structurally heterogeneous compounds (Gupta et al., 1993, Vaccine, 11: 293-306). Classically recognized examples of adjuvants include oil emulsions (e.g., Freund's adjuvant), saponins, aluminium or calcium salts (e.g., alum), non-ionic block polymer surfactants, lipopolysaccharides (LPS), mycobacteria, tetanus toxoid, and many others. Theoretically, each molecule or substance that is able to favor or amplify a particular situation in the cascade of immunological events, ultimately leading to a more pronounced immunological response can be defined as an adjuvant.
A galactosylceramide (α-GalCer) is a glycolipid, more specifically a glycosylceramide, originally isolated from Okinawan marine sponges (Natori et al., Tetrahedron, 50: 2771-2784, 1994), or its synthetic analog KRN7000 [(2S,3S,4R)-1-O-(α-D-galactopyranosyl)-2-(N-hexacosanoylamino)-1,3,4-octadecanetriol], which can be obtained from Pharmaceutical Research Laboratories, Kirin Brewery (Gumna, Japan) or synthesized as described previously (see, e.g., Kobayashi et al., 1995, Oncol. Res., 7:529-534; Kawano et al., 1997, Science, 278: 1626-9; Burdin et al., 1998, J. Immunol., 161:3271; Kitamura et al., 1999, J. Exp. Med., 189:1121; U.S. Pat. No. 5,936,076).
It was shown that α-GalCer can stimulate natural killer (NK) activity and cytokine production by natural killer T (NKT) cells and exhibits potent antitumor activity in vivo (Kawano et al., 1998, Proc. Natl Acad. Sci. USA, 95:5690). After intake by antigen presenting cell (APC), which is represented by dendritic cell (DC) and the like, α-galactosylceramide is presented on the cellular membrane by a CD1d protein similar to major histocompatible complex (MHC) class I molecule. NKT cells are activated by recognition using TCR (T cell receptor) of the thus-presented complex of CD1d protein and α-galactosylceramide, which triggers various immune reactions. Invariant Natural Killer T cells have been also shown to induce B cell activation, enhancing B cell proliferation and antibody production (Galli et al, Vaccine, 2003, 21: 2148-S2154; Galli et al, J Exp. Med, 2003, 197: 1051-1057).
These studies open the possibility that α-GalCer may play an equally important role in bridging not only innate immunity mediated by NKT cells, but also adaptive immunity mediated by B cells, T helper (Th) cells and T cytotoxic (Tc) cells. Recently, α-GalCer has been shown to act as an adjuvant for a variety of co-administered protein antigens and saccharide antigens (W003/009812).
The development so far exhibits the simultaneous use of the vaccine and an adjuvant that produces the desired immunogenicity. A major drawback of protein-based vaccines, where a conjugation of antigenic carbohydrates to proteins is required, is that the vaccine is particularly heat unstable and a refrigeration of the vaccine is required. Moreover the use of at least two components to achieve a sufficient vaccination is also a significant drawback, since the procedure of administration is rather complex, e.g. the point in time where the adjuvant is administered is essential to achieve the desired immunogenicity (WO003009812).